Method of producing corrosion resistant beryllium bodies



United States Patent 3,410,768 METHOD OF PRODUCING CORROSION RESISTANTBERYLLIUM BODIES Louis J. Csontos, Parma, and Albert James Stonehouse,Lyndhurst, Ohio, assignors to The Brush Beryllium Company, Cleveland,Ohio, a corporation of Ohio No Drawing. Filed Sept. 8, 1964, Ser. No.395,012 14 Claims. (Cl. 204-37) ABSTRACT OF THE DISCLOSURE A process forproviding a corrosion resistant beryllium body by anodizing said body inan electrolytic bath containing sodium chromate and chromic acid atpredetermined temperature and hydrogen ion concentration parameters,subsequently heat treating said body, and beryllium bodies and bodieshaving beryllium surfaces provided by said process having corrosionresistance under specified environments.

This invention relates generally to beryllium bodies having a corrosionresistant surface coating, and to a method for application of such acoating.

More specifically, the invention relates to anodized beryllium bodieshaving a corrosion resistant surface coating, and to a method foranodizing the surface of said bodies.

Beryllium metal has become well established for applications in inertialguidance systems, structural compo- .nents for aircraft and aerospaceuse, and in other fields previously developed methods have failed toprovide the desirable combination of properties obtained by the presentmethod.

The present method provides corrosion resistant beryllium materialswhich greatly extend the application of beryllium in various fields, andwas developed as a result of extensive investigation.

A primary object of the invention is to provide anodized corrosionresistant beryllium bodies,

A further object is to provide beryllium bodies resistant to corrosionin high temperature air, water, deionized water, and salt sprayenvironments.

A still further object of the invention is to provide a method forapplying a corrosion resistant coating to the beryllium surface ofbodies.

Another object of the invention is to provide a method for anodizing theberyllium surfaces of bodies to render them corrosion resistant to hightemperature, water, deionized water, and salt spray environments.

Other objects and advantages of the invention will become apparent fromthe following description.

The method comprises applying a protective surface coating to berylliumsurfaces of bodies by passing an electric current to the berylliumsurface connected as an anode, in a bath consisting essentially ofwater, sodium chromate in an amount of at least 0.4%, by weight, of thebath, and an amount of chromic. acid sufficient to adjust the pH rangeof the bath from about 5.5 to about 7.5, preferably to about 7.0.

The concentration of the sodium chromate solution ranges generally fromabout 0.4% to about 10%, by weight of the bath, and preferably is usedat a 5% weight concentration. The use of sodium chromate in excess of10% does not appear to increase the effectiveness of the method orcoating appreciably.

The bath should be free of hydrochloric and sulfuric acids, as theirpresence prevents formation of the desired protective coating.

The temperature of the bath must be maintained below about 130 F., sincethe coating applied above this temperature does not afford properprotective resistance. The temperature of the bath should be less than130 F.,

TABLE Ir-PROCESS VARIABLES Current Wt. Cone. Hours Tested PhysicalCondition Example Voltage Denslty, Time (mm) of Sodium pH Temperature in800 C. of Specimen After No. Amps/ft. Chromate, of Bath in F. Dry AirTest Exposure percent Effect of Anodizing Time:

12 170 10 5 7.0 75 96 No attack or Failure.

20 5 7. 0 75 96 Do. 30 5 7. 0 75 96 Do. 5 7. 0 75 96 Do. 5 7. 0 75 96D0. 5 7. 0 96 Do.

60 5 7. 0 75 96 Do. 60 5 7.0 75 96 D0. 60 5 7. 0 75 96 D0. 60 5 7. 0 7596 Do. 298 60 5 7. 0 75 96 Do. ation: 166 1 7.0 75 161 Do. 160 60 2 7.075 161 Do. 60 5 7.0 75 161 Do. 60 10 7.0 75 161 Do.

10 1. 5 7. 0 75 282 Do. 10 0. 6 7. 0 75 282 Do. 10 1 7.0 75 137 Do. 300. 4 7. 0 75 96 Hours tested in 1489 F.- 1,659 F Dry Air Efiect ofTemperature Variation:

12 160 60 5 7. 0 176 65 Failed, Corroded. 12 160 60 5 7. 0 65 D0. 12 605 7. 0 75 203 No Attack or Failure. 12 160 60 5 7. 0 129 174 Do.

ranging from about 70 F. to about 110 F., and is preferably about 75 F.

For optimum resistance of the protective coating, the solution of sodiumchromate should be prepared with distilled water and the pH adjustedwith reagent grade chromic acid. The current density may vary from about48 to about 300 amperes per square foot, and is preferably maintained atfrom about 150 to about 200 amperes per square foot. The current isapplied for a period of time ranging from about minutes to about 60minutes.

Examples of beryllium bodies protectively coated by the method of theinvention and exposed to high-temperature air environment are presentedin the preceding Table I. In each example, a beryllium coupon 0.50 wide,1.25" long and 0.1875" thick was protectively coated under the variousprocessing conditions set forth in Table I.

Table II shows the effect of exposure to various testing environments ofbodies protectively coated at the process conditions specified in thetable.

surface of the base metal, the overall dimensional change is less than0.0001 per side, during application time of up to minutes. The smalldimensional change in the surface is the result of replacement of theremoved beryllium by a protective coating, the volume of which is approximately equivalent to that of the removed metal.

Thus, the negligible dimensional change which occurs in the surfaceduring application of the protective coating is unexpected, as berylliumoxide is known to occupy a larger volume than the amount of berylliummetal stoichi ometrically required to produce a given quantity of oxide.The final surface of the protectively coated body is identical insurface pattern with the original body surface, as the degree of polish,machining marks, and blemishes are faithfully reproduced.

The coatings applied by the method of the invention are quite dense andrelatively thick. Generally, the thickness of the coating varies from0.1-2.4 mils, but preferably is about 0.5 mils, dependent upon thecontrol of the process conditions.

TABLE. II. EXPOSURE TESTS OF ANODIZED BERYLLIUM BODIES Example N0.Beryllium Body Size Test Environment Dur i tion of Condition of Bodiesest 1 1" high x A" Dia 800 C. in 1% Moist Air 1,749 Hrs No attack orfailure of anodic coating.

2 nxtxis' 850 0.1m dry, CO2 Free Air 185 Hrs... Do.

3 154 x 55 x M 900 C. in dry, CO1 free air 137 IIrs Do.

4 1,000 C. in dry, CO2 free air. 137 Hrs D0.

5 1.100 C. in dry, CO2 free air 113 Hrs. Do.

6 700 C. in dry 02 Hrs No attack or failure of anodic coating.

7 16 x x as 817 C. in dry 01 60 Hrs No attack or failure of anodiccoating.

Wt. gain=0.33 trig/cm).

8 X x 1.030 C. in dry 02 No attack or failure of anodie coating. Wt.gain =0.58 mgJcmfl.

9 thick x 2-is' Dia 5% NaCl Salt Spray Hrs No attack or failure ofanodic coating.

20% NaCl Salt Spray 24 Hrs Do. 110 F. deionized water 1,656 Hrs... Do.Tap Water 2,160 Hrs Do. 0.1 N NaCl Soln 336 Hrs D0.

1 All specimens included in Table II were anodized in a bath containing5 weight percent sodium chromate with the pH adjusted to 7.0 withchromic acid at a temperature of 75 F., 12 volts and a current densityof 160 It will be noted from Tables I and II that all the bodies withthe exception of those protectively coated in Examples 21 and 22 ofTable I at a bath temperature above F., were completely protected fromoxidation. No failure of the protective coating other than a slightchange in color occurred, during test exposure of body specimens of allthe other examples. Commercial beryllium when exposed to the sameconditions normally oxidizes after 48 hours exposure to the extent that25 to 50% of the test pieces will have been reduced to a fluffy powderedoxidation 'product.

A comparison exposure test of commercial beryllium specimens with theanodized bodies of Examples 6-8 inclusive, of Table II, showed thatafter exposure in one atmosphere of dry oxygen at 705 C. for 75 hours,807 C. for 45 hours, and 1030 C. for 35 hours, the commercial specimensincurred a weight gain of 17.0, 63.0, and 37.0 mg./cm. respectively, apronounced increase in weight compared to the weight gained by bodies ofthe examples even though the commercial specimens were tested for ashorter time at approximately the same temperatures. The examples thusillustrate the high corrosion resistance of the beryllium bodies, thesurfaces of which are coated in accordance with the method of theinvention.

The examples are presented as illustrative embodiments of the inventionand include the preferred embodiments known at the time of preparationof the specification.

Although the method of applying the protective coating to berylliumbodies results in some dissolution of the The protective coating appliedis believed to be a form of hydrated beryllium oxide as it is nearlyamorphous in the initially deposited form, as indicated by X-rayanalysis. When heated at elevated temperature for a sufficient time, theinitial coating is converted to normal crystalline beryllium oxide, asevidenced by X-ray diffraction analysis. Temperatures of about 800 C.may be employed for efiFecting conversion to the crystalline state.

Having thus described our invention, we claim:

1. The method of applying a protective surface coating to a berylliumsurface of a body, comprising: passing an electric current to thesurface with the surface connected as an anode and immersed in a bathconsisting essentially of water, sodium chromate in an amount at least0.4%, by weight, of the bath, and an amount of chromic acid sufficientto adjust the pH of the bath into a range from about 5.5 to about 7.5,the temperature of the said bath being below about 130 F.

2. The method according to claim 1 wherein the sodium chromate rangesfrom about 0.4% to about 10%, by weight, of the bath, and a sufficientamount of chromic acid is present in each concentration of sodiumchromate to maintain the pH of the bath from about 5.5 to about 7.5.

3. The method according to claim 1 wherein the electric current densityis from about 48 to about 300 amperes per square foot.

4. The method according to claim 1 wherein the electric current isapplied from about 10 minutes to about 60 minutes.

5. The method according to claim 1 wherein the water is distilled waterand the pH is adjusted by reagent grade chromic acid.

6. The method according to claim 1 wherein the bath temperature rangesfrom 70 F. to about 110 F.

7. The method according to claim 1 wherein the body consists essentiallyof beryllium.

8. The method according to claim 1 wherein, after application, thecoating is heated to a temperature sufficient to convert the coating tocrystalline beryllium oxide.

9. The method according to claim 8 wherein said temperature is about 800F.

10. A method of applying a protective surface coating to beryllium,comprising: passing an electric current having a density of from about150 to about 200 amperes per square foot to a body having a berylliumsurface connected as anode and immersed in a bath consisting essentiallyof water, about 5% sodium chromate, by weight of said bath, and anamount of chromic acid sufficient to adjust the pH of the bath to about7.0, the temperature of said bath being about 75 F.

11. The method according to claim 10 wherein the water is distilledwater and the pH is adjusted by reagent grade chromic acid.

12. A bath for the electrolytic production of protective corrosionresistant coatings on beryllium surfaces, consisting essentially ofwater, sodium chromate in a concentration ranging from about 0 .4% toabout 10%, by

Weight of the bath, and adjusted by an addition of chromic acid to a pHof about 7.0, said bath being free of hydrochloric and sulfuric acids,respectively.

13. A bath according to claim 12 wherein the concentration of the sodiumchromate is approximately 5%.

14. A bath according to claim 12 wherein the water is distilled water,and the chromic acid is reagent grade.

References Cited UNITED STATES PATENTS 2,606,866 8/1952 Neish 204562,871,425 1/1959 Burnham 204-56 X 3,276,974 10/ 1966 Tyson 204-56 XOTHER REFERENCES Levin, M. L.: The Formation of Crystalline Anodic OxideFilms on Beryllium-Transactions of The Faraday Society, vol. 54, pp.935-940, 1958.

Whitby, L, et al.: Chromic Acid Anodizing of BerylliumProceedings of TheAmerican Electroplaters Society, col. 48, pp. 106408, 1961.

Missel, Leo: Chromic Acid Anodizing of Beryllium-- Proc. of The AmericanElectroplaters Society, vol. 48, pp. 109-111, 1961.

HOWARD S. WILLIAMS, Primary Examiner. G. KAPLAN, Assistant Examiner.

